Thoracic lymphangiomatosis in childhood is a rare disorder that frequently has poor response to medical therapy as well as a poor prognosis. This disease is often misdiagnosed, and a definitive diagnosis is usually delayed because of the rarity and slow course of the disease. No previous reports have so far described the usefulness of lymphoscintigraphy in the diagnosis of lymphangiomatosis, although some authors have reported the efficacy of lymphoscintigraphy for evaluating chylothorax. A 6-year-old boy presented with a diffuse mediastinal mass and received an open mediastinal biopsy for a definitive diagnosis, which led to the occurrence of massive chylothorax postoperatively. A diagnosis of lymphangiomatosis was finally made based on the lymphoscintigraphic findings demonstrating an obstruction of the thoracic duct. This report describes a rare case of thoracic lymphangiomatosis diagnosed at autopsy, and suggests that the minimally invasive technique of lymphoscintigraphy should be employed to obtain a definitive diagnosis at an early stage.